An ultrasound diagnosis apparatus insonifies ultrasound pulses to a patient or an object (hereinafter referred to as a specimen). The ultrasound pulses are generated from transducers built in an ultrasound probe. The ultrasound diagnosis apparatus then receives echo signals from the specimen with the ultrasound transducers. The echo signals occur due to a difference of acoustic impedances among tissues of the specimen.
Such a diagnosis technique mentioned above requires easy operations, such as contacting the ultrasound probe with a surface of a body of the specimen. The operations are usually performed by a doctor or the like (hereinafter referred to as an operator). Therefore, the operator can easily observe two-dimensional ultrasound images in real time. The diagnosis technique is widely used for functional and/or morphological diagnoses of organs, such as a heart. Particularly, in an ultrasound diagnosis for an area around the heart, it is very important to evaluate heart functions objectively and quantitatively. Therefore, items to be measured in the diagnosis for the heart usually include a kinetic rate of heart tissues, a speed of blood stream, and an area and/or a volume of heart chambers.
When a motor function of the heart is diagnosed, images are displayed as a moving image and it is preferably desired to make the diagnosis on the basis of three-dimensional information. In order to meet such a clinical desire or requirement, it is expected to put a real-time three-dimensional scan technique into a practical use in the future. Currently, however, it does this with a plurality of two-dimensional moving images acquired from respective different directions against the heart. Images included in the respective moving images obtained in the above manner are tried to be displayed in a time phase adjusted manner in a single display. The time phase can be defined as time of image acquisition in repeated cycles of heartbeats. One example requiring a time phase adjustment may be a simultaneous display of tomograms along a major axis of the heart and tomograms along a minor axis of the heart. Another example may be a simultaneous display of a moving image of a predetermined part of the heart under a normal condition and a moving image of the predetermined part immediately after an exercise stress has been given to the specimen. The other example mentioned above may be called an exercise stress echocardiography.
Further, there is a technique of measuring a volume of a heart chamber based on two tomograms, which are orthogonal with each other. For example, four-chamber view image data and two-chamber-view image data are acquired in a form of a moving image, respectively. In this case, the volume of heart chambers is measured on the basis of the image data according to the measurement technique. The four-chamber view image may represent a tomogram showing two atrials and two ventricles of the heart. The two-chamber view image may represent a tomogram showing one atrial and one ventricle of the heart.
In an ultrasound diagnosis technique with an intent to examine such heart functions, it is important to adjust a time phase of cardiac pulsation (heart strokes) between two moving images. This is particularly important in the event of displaying two moving images each of which are under a different imaging condition or in the event of calculating a volume based on such two moving images. Hereinafter, sequential image data (time-series image data) acquired as a moving image or picture are referred to as sequential images or sequential image data.
In the above ultrasound diagnosis technique, it generally utilizes a heartbeat synchronous technique wherein, for example, electrocardiographic complex information is acquired during the acquisition of the ultrasound images. Alternatively, for example, the ultrasound images are sequentially acquired in synchronization with R-waves of an electrocardiographic complex. According to the former case, ultrasound image data are acquired with an electrocardiographic complex under each of different conditions (e.g., a four-chamber view image acquisition and a two-chamber view image acquisition). When these image data are reproduced and displayed, image data obtained under the four-chamber view image acquisition and image data obtained under the two-chamber view image acquisition are sequentially displayed, for example, side by side in a single display. Each of those image data is for an image acquired in every predetermined time period after the R-wave has occurred. Further, various measurements including a volume calculation of the heart chamber are made based on these image data. In this regard, when an image in a predetermined time phase is selected, it is also quite common to set an image number (or a frame number) of the ultrasound images so as to determine the selected image, instead of setting a time period elapsed from the R-wave occurrence. The frame number is determined on the basis of time instant of the R-wave occurrence.
As described above, application of a heartbeat synchronous technique to two different kinds of sequential images enables the display of two heart images in time phase, each of which under a different condition. This has resulted in a great improvement in measurements of the heart function by ultrasound pulses.
When, however, a time phase of the ultrasound images is determined (or set) based on the electrocardiographic complex in a conventional manner, intervals between R-waves of an electrocardiographic complex may not always be constant. Particularly, specimens to undergo cardiac examinations are likely to suffer from an arrhythmia. Further, even if it is a normal healthy person, it is obvious that intervals between R-waves of the electrocardiographic complex are outstandingly short after the exercise stress has been given to the person. Still further, it is known that intervals between R-waves of the electrocardiographic complex are not regularly short and long in some cases of a heart disease, but rather, for example, different in proportions between a systolic period and a diastolic period.
Drawbacks of the conventional technique will be described in FIG. 1 in an exemplary case that a diastolic period of the electrocardiographic complex varies temporally regarding a predetermined specimen. FIG. 1 is an illustration showing a relationship of sequential images between two predetermined R-wave intervals of an electrocardiographic complex according to a prior art of the present invention. In FIG. 1, FIG. 1(a) shows the electrocardiographic complex. FIG. 1(b) shows image numbers of sequential images in two different predetermined time periods (or frame numbers of ultrasound images in the two predetermined R-wave intervals). FIG. 1(c) shows volumes of a heart chamber in the sequential images. For example, N0+1 images of a four-chamber view (a first image series) are sequentially acquired during an interval between an R-wave R1 and an R-wave R2 (hereinafter referred to as an interval R1–R2) of the electrocardiographic complex. Also, N0 images of a two-chamber view (a second image series) are sequentially acquired during an interval between an R-wave R3 and an R-wave R4 (hereinafter referred to as an interval R3–R4) of the electrocardiographic complex. In the interval R1–R2, a first image (image 1) may be acquired in a time t1 after the R-wave R1 occurrence. A second image (image 2) may be acquired in a time t2 after the R-wave R1 occurrence. Similarly, an N0th image (image N0) may be acquired in a time tN0 after the R-wave R1 occurrence. In FIG. 1(c), a period from a peak to a valley in each of the intervals R1–R2 and R3–R4 may be called a systolic period. Further, a period from a valley to a peak in each of the intervals R1–R2 and R3–R4 may be called a diastolic period. A time of the peak may be called an end-diastolic time. A time of the valley may be called an end-systolic time.
When it comes to an exercise stress echocardiography, images of the specimen before an exercise may be acquired, for example, during the interval R1–R2. Similarly, images of the specimen after the exercise may be acquired, for example, during the interval R3–R4.
Usually an image acquisition time is almost constant for any one of ultrasound images. Therefore, when a diastolic period in the interval R3–R4 is shorter than a diastolic period in the interval R1–R2, an end-diastolic time Q1 in the interval R1–R2 may correspond to an (N0+1)th image (or frame) of the first image series while an end-diastolic time Q2 in the interval R3–R4 corresponds to an N0th image (or frame) of the second image series. As described in the above example, when a diastolic period and/or a systolic period temporally varies, it is difficult to properly comprehend a time phase or a relationship between the first image series and the second image series if such a time phase is interpreted in accordance with an image obtained in a predetermined time after a respective R-wave occurrences in an electrocardiographic complex. It is also difficult if such a time phase is interpreted in accordance with an image number (or a frame number) of images included in a respective predetermined R-wave interval. Therefore, it causes difficulties in a time phase adjusted display and/or various kinds of measurements in time phase based on the first image series and the second image series.